This invention relates generally to traffic control systems, and more particularly to improvements in operating traffic signal lights at controlled roadway intersections during times when loss of electrical power occurs.
At the present time traffic control systems use a controller unit that energizes load switches that drive the signal lamps through a flash transfer relay. In the event that a conflicting signal should arise, a conflict monitor actuates the relay to transfer the traffic signal loads to a flasher module. When this transfer occurs, the controller unit and load switches are removed from causing the traffic signal lights to be turned ON and to be turned OFF. Once the relay is actuated to transfer the traffic signal loads to the flasher module, human intervention is required to restore the flash transfer relay to the state where the controller unit and load switches can cause the traffic signal lights to be turned ON and to be turned OFF, thereby removing the flasher module from operating the traffic signal lights.
The flasher module is capable of causing the traffic signal lights to alternate regularly OFF and ON. This is accomplished by the flasher module in such manner that traffic signal lights are flashed ON and OFF. In doing so, drivers of vehicles may see flashing red traffic signal light indications at the intersection, indicating for them to stop before proceeding through the intersection in a safe manner.
A traffic control system is normally considered as consisting of a traffic controller unit for the purpose of providing 24 volt DC input signals to one or more load switches used to turn traffic signal lights ON. A conflict monitor device is used to monitor the presence of proper alternating current field wire voltages supplied to power the traffic signal lights. When improper AC voltages exist, the conflict monitor causes an electro-mechanical relay to operate, which in turn causes the high current capacity flash transfer relay to remove traffic signal light power from the load switches and to connect the traffic signal light power to a flasher unit, which causes traffic signal lights to flash ON and OFF.
Operation of a traffic control system described above requires the supply of AC power to equipment. When AC power ceases to be supplied, the traffic control system ceases to operate and the traffic signal lights no longer emit light, thereby becoming dark. The result is that drivers of vehicles approaching the signalized intersection do not see any traffic signal lights. The drivers of vehicles approach what is typically referred to in the industry as “a dark intersection”. The Manual on Uniform Traffic Control Devices (MUTCD) of the Federal Highway Administration (FHWA), listed in the Federal Register, states that it is acceptable to operate a signalized intersection as “a dark intersection” and that, during such operation, drivers of vehicles are expected to interpret “a dark intersection” the same as they would an intersection having stop signs; stopping their vehicles before proceeding through the intersection.
One of the purposes of traffic signals is to make intersections more visible, and hence, safer. There has long been need for improvements in making “a dark intersection” more visible to drivers of vehicles for traffic control. Traffic signal lights may be difficult to see even when operating properly and lit. But when traffic signal lights are dark, intersections become very difficult for drivers of vehicles to see. The result is that accidents occur, causing property damage and bodily harm with potential loss of life. Power outages leading to loss of traffic signal light operation are most likely to occur as the result of inclement weather, which causes visibility to be degraded. Thus, the loss of traffic signal operation most commonly occurs when its reliable operation is needed most. Loss of traffic signal light operation during nighttime due to loss of power poses an all-too-common threat to the safety of drivers of vehicles, their passengers and bystanders.
Back-up power supplies with power storage capability have been used in traffic control systems at signalized intersections to maintain operation of the traffic control system as it would operate from the external AC power source. Use of such back-up power sources has been limited to only a few signalized intersections, due to space limitations and their high cost.